Method of treating silicon steel sheets



Patented July 19, 1932 UNITED STATES PATENT OFFICE Eowmn'. mmimmb, or rmsnuaen, rnnivsnvmm unfrnon or TREATING ,smrcon STEEL snnnrs No Drawing. Application fled April 2, 1929, Serial No. 352,051. Renewed October 30, 1931.

My invention relates to the manufacture of steel sheets, and primarily to a new and useful method for the production out of alloys of iron and silicon,-or iron, silicon and aluminum, of sheets suitable for use as laminae of electrical apparatus having alternating magnetic fields, as for example armature cores, and more particularly transformer cores.

1t) It has long been the practice to build up cores for these apparatus of layers or laminae of sheet metal of special allo s to produce a core or analogous element 0 increased electrical elficiency, i. e., a high degree of magnetic permeability and a low degree of magnetic hysteresis. It is also well known that sheets of iron-silicon or iron-siliconaluminum alloys constitute a superior material for these purposes. With this knowledge as a starting point, numerous attempts have been made still further to improve the electrical efiiciency' of these so called silicon steels by combining the several constituent elements "in varying proportions, by subjecting the alloys thereby produced to different details of heat and mechanical treatment, and p in other ways- I have found that sheets of these alloys prepared according to the process which I 8 have invented, and which I shall proceed to describe and claim in this application for letters patent, exhibit, in addition to satisfactory physical qualities, electrical properties superior to those possessed by similar alloy sheets treated according to any process heretofore known, so far as I am aware. More specifically, the improvement in silicon steel sheets produced according to my present invention consists of a decrease in energy loss, or watt loss, of approximately fifteen percent, and anincrease in magnetic permeability of about ten percent.

The metal upon which my new process can be practiced with greatest success is commonly known as silicon steel? The material with which I have obtained' best'results is iron with which has beenialloyed silicon in the proportion of two to six percent or more, containing the usual trace of carbon unavoid- ,able in commercial ferrous alloys, though I prefer to limit the proportion of this element to the lowest practical degree, and containing also, if desired, a small amount of aluminum. It should be borne in mind in construing the appended claims that the term silicon steel used therein and in this specification is intended to include any and all the alloys of iron and silicon, or iron, silicon and aluminum, and their equivalents.

A- typical final sheet thickness is four- 80 teen mils, but it should be noted that sheets of any desired thickness may be made by my method, the essential features of which consist not in producing a sheet of any particular size or shape, but in subjecting suitable materials, as hereinabove specified, to a series of heat treatment and mechanical treatment steps, which will now be described in detail.

. An ingot of silicon steel isfirst reduced to sheet bar stock by any convenient method, 10 preferably by hot rolling. These bars are then rolled into sheets, which necessitates reheating the now cooled bar stock to a suitable rolling temperature, i. e., about 1500 F.,. and giving them a suiiicient number of passes throu h hot mill rolls to,reduce them to the seminished sheets known .as breakdowns. During this operation the temperature has of course fallen, so that before beginning the next rolling the break-downs 0 i are reheated to a suitable rolling temperature, which,-as has been said is approximate- 1y 1500 F. The break-downs are then further rolled to dimensions which will depend on thosedesired in'the ultimate finished sheet. so In this condition the sheets are known as run-overs and their temperature has again dropped, as after the first rolling operation resulting in the break-downs, to 1100- to 1400 F. I

The run-overs are then doubled by folding over lengthwise on themselves and heated to the rolling temperature of about 1500 F. after which they are subjected to a series of final rolling operations so vconductedfthat,- v when comp eted, sheets of the size ultimately desired will be produced.

The manner in which'these last rolling operations are performed is of the utmost importance. Any and all of'the preceding in the manner in which the strain is introduced,

whether by rolling, pressing or in any other way, is immaterial, and this should be understood in construing the appended claims. I shall describe herein one manner of achieving'the desired strain which 1 have found ,eminently practical and satisfactory, i. e., a seriesof particular kinds of rolling operations, but my invention is by no means limited thereto.

One manner of conducting these final steps is therefore as follows:

The run-over sheet, having b en heated to the rolling temperature of a proximately 1500 F., is given about four passes through the rolls in such a way that the last pass is made when the sheet has cooled to just below the lowest critical temperature (the point corresponding to Ar in plain carbon steel) for sheets of this character, i. e., the last pass is made at about 1250 F. It is by no means necessary that the number of passes constituting this step should be exactly four. This number is mentioned because it is a convenient and economical number. Under vary conditions and circumstances fewer or more passes may prove exped1ent. Whatever the number of passes constituting this step in my process, it is necessary that the last pass be made when the sheet is at the temperature indicated.

The sheet is next sheared to pattern size and box annealed at a temperature of about 1300 F. This is approximately temperature at which I have found this annealing step may be successfully practiced. Higher temperatures, while not harmful, are not necessary. v

The annealed sheet having been allowed to cool to room temperature, the essential step of strain introduction, characteristic of my process, is practiced. A preferred manner ,of achieving the strain introduction consists in passing the sheet through cold rolls a sufficient number of times to produce an elongation of approximately one percent. I have found that sheets elongated to this extent possess the requisite strain and consequently exhibit the desired improvement in electrical properties, but as a factor of safety, in practice Iprefer to subject the sheets to an elongation of slightly more than one percent, say one-and-one-quarter percent, the additional one-quarter percent apparently not adversely effecting the superior electrical qualities induced in the sheet by the one percent elongation.

Lastly, the sheets are reannealed at 1600 F. or higher.

I have found that the hot rolling step hereinabove described may, if desired, be omitted without entirely sacrificing the beneficial results of the strain introducing operation on the cooled sheet. The latter is the essential step of my process, and as pointed out in the appended claims may, to the exclusion -of the other steps herein explained, constitute one manner of practicing my present invention. For best results, however, I prefer to subject thesheets first to the hot rolling operation as just described.

The final thickness of the finished sheet is commonly about fourteen mils. I Since the elongation of the sheet, hereinbefore described, is only one or one-and-one-quarter percent, the sheet just prior to the elongating passes, which may be as many as four in numher, is too thin to be conveniently rolled one sheetat a time. I have therefore found it expedient to roll two or more sheets together. The object of passing two or more sheets through the cold rolls at one time is to give elongation. I have found that the association of a plurality of sheets during a pass through the rolls in no way interferes with the uniform elongation of each sheet. While the number of passes recommended for this elongating operation is four, a smaller number of passes may suffice, depending upon the silicon content ofthe alloy used and upon the pressure of the rolls.

Having thus described my invention, what I now claim and desire to secure by Letters Patent is: v

- 1. The method of treating silicon steel to improve its electrical properties, which consists in elongating the steel approximately 1% by cold rolling, and then annealing at a temperature of approximately 1600 F.

' 2. The method of treating silicon steel to improve its electrical properties, which consists in rolling the steel at a temperature just below its lowest'gritical temperature, then annealing, then --.by cold working introducing suflicientstrain into the steel so that after re-annealing its magnetic permeability will be increased, and then re-annealing the steel.

3. The method of treating silicon steel to improve its electrical properties, which coning its ma etic permeability will be increased, an then re-annealing the steel.

4. The method of treating silicon steel to improve its electrical properties, which consists in rolling the steel at a temperature just below its lowest critical temperature, then annealing, then cold rolling to produce an elongation of approximately 1%, and then re-annealing the steel.

-5. The method of treating silicon steel to improve its electrical properties, which consists in subjecting the steel to a plurality of passes through hot rolls so that the last pass is made at a temperature just below the lowest critical temperature, then annealing, then cold rolling to produce an elongation of approximately 1%, and then re-annealing the steel. I

6. In the method of treating silicon steel to improve-its electrical properties, the steps comprising cold rolling the steel to effect an elongation of approximately 1%, and then annealin the steel.

7. In t e method of treating silicon steel to improve its electrical properties, the steps comprising rolling the steel at a temperature just below the lowest critical temperature, then annealing the rolled steel, then by cold working introducing strain into the annealed steel, and then annealing the steel.

8. In the method of treating silicon steel to improve its electrical properties, the steps comprising subjecting the steel to a plurality of passes through hot rolls, so that the last pass is made at a temperature just below the lowest critical temperature, then annealing the rolled steel, then by cold working introducing strain into the steel, and then anneal ing the steel.

9. In the method of treating silicon steel to improve its electrical properties, the steps comprising rolling the steel at a temperature just below its lowest critical temperature then annealing the rolled steel, then cold rolling the annealed steel to produce an elongation of approximately 1%, and then-am nealin the steel at a temperature higher than t e first annealing temperature.

ll). In the method of treatin silicon steel,

the steps, comprising cold wor ing the steel.

to effect a decrease in thickness sufiiciently to introduce strain into the steel to the extent required so that after annealing the mag-- netic permeability will be increased up to 10% and the watt loss decreased up to 15%, and then annealing the steel.

In testimony whereof I afiix my signature.

EDWARD M. FREELAND. 

